Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, referred to as an organic EL element). An inorganic electroluminescent element has been utilized as a flat light source, however, requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescent element is an element provided with a constitution comprising an emission layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emission layer to be recombined, resulting emission utilizing light release (fluorescence-phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
In an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption. Examples of such technologies are a slight amount of a fluorescent substance doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element (for example, refer to Patent Document No. 1). Further, there are known such as an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance (for example, refer to Patent Document No. 2). and an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, refer to Patent Document No. 3).
Regarding to the technologies disclosed in the above-described Patent Documents, when emission from an excited singlet is utilized, since a generation ratio of a singlet exciton to a triplet exciton is 1/3, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of a quantum efficiency (ηext) of taking out is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (for example, refer to Non Patent Document 1), researches on materials exhibiting phosphorescence at room temperature have come to be active (for example, refer to Non Patent Document 2). Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet, which is principally 4 times of the case of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application. As a dopant used for an organic electroluminescent element employing phosphorescence, many compounds mainly belonging to iridium complexes have been investigated (for example, refer to Non Patent Document No. 3).
An example is tris(2-phenylpyridine)iridium (refer to Non Patent Document No. 2). In addition to that, there have been studied to use L2Ir(acac) such as (ppy)2Ir(acac) as a dopant (for example, refer to Non Patent Document No. 4). Also there have been studied to use compounds such as tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3) and tris(benzo[h]quinoline)iridium (Ir(bzq)3), Ir(bzq)2CIP(Bu)3 (for example, refer to Non Patent Document No. 5). Also known are investigations on iridium complexes having phenyl pyrazole as a ligand (refer to Non Patent Documents No. 4).
A FIrpic, which is a typical phosphorescent blue dopant, has achieved a shortening of emission wavelength with a fluorine-substituted phenylpyridine and a picolinic acid being employed as a major ligand and an auxiliary ligand respectively. As an auxiliary ligand, other than the picolinic acid, a pirazabole type ligand has been known to shorten an emission wavelength by being introduced into the dopant. (for example, refer to Patent Document 1 and Non-Patent Documents 1 and 2). The aforesaid dopants, in combination with carbazol derivatives or triarylsilanes as a host compound, have realized high efficient elements, but on the other hand, have significantly degraded emission life of the elements. Then, improvement to overcome the trade-off relationship has been demanded.
Any of the above-described blue dopants are such types of compounds that the dopant materials have a low level of the highest occupied molecular orbital (hereinafter abbreviated as “HOMO”) and the lowest unoccupied molecular orbital (hereinafter abbreviated as “LUMO”). HOMO and LUMO levels of the above dopants are lower by about 1 eV compared to those of a Ir(ppy)3, which is a typical phosphorescent green dopant. As a blue dopant, compounds having low HOMO and LUMO levels have been known, but only a few compounds having high HOMO and LUMO levels have been reported. Recently, blue dopants having high HOMO-LUMO levels were reported (refer to Patent Documents 5 and 6), but these dopants are insufficient with regard to efficiency and life, therefore the achievements of higher efficiency and longer life are the issues.
Patent Document 1: Japanese Patent Registration No. 3093796
Patent Document 2: Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 63-264692
Patent Document 3: JP-A No. 3-255190
Patent Document 4: WO 04/085450
Patent Document 5: U.S. Patent Registration No. 2004/0048101
Patent Document 6: WO 04/085450
Non-Patent Document 1: M. A. Baldo et al., Nature, Vol. 395, pages 151-154 (1998)
Non-Patent Document 2: M. A. Baldo et al., Nature, Vol. 403, No. 17, pages 750-753 (2000)
Non-Patent Document 3: S. Lamansky et al., J. Am. Chem. Soc., Vol. 123, page 4304 (2001)
Non-Patent Document 4: M. E. Tompson et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu)
Non-Patent Document 5: Moon-Jae Youn. Og, Tetsuo Tsutsui et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu)